I. Field of the Invention
The present invention relates to a method for densifying previously sintered parts of powdered metals, ceramics, hard metals and the like which are sintered in the presence of a liquid phase.
II. Description of the Prior Art
In the liquid phase of sintering powdered metals, ceramics, hard metals and the like, the powdered material which may comprise a powdered hard phase and powdered metal binder, is first intermixed with a fugitive binder which holds the part in the desired shape after cold pressing. Usually this fugitive binder consists of paraffin, polyethelene glycol, or a metal containing hydrocarbon or other organic plasticizer. The cold pressed part is conventionally known as a preform.
The preforms are then subjected to a presintering step in which the preforms are slowly heated, thus vaporizing the fugitive binder and the vaporized binder is removed from the part by a wash gas, vacuum pumping or other means. Following the presintering step, the parts retain their shape due to some solid state sintering of the powdered binder.
The parts are then subjected to a sintering operation in which the parts are raised to a temperature where some liquid phase appears and some solutioning of the hard phase occurs. This appearance of the liquid phase relatively rapidly densifies the part due to capillary action and solutioning effects. Following the sintering of the parts, the parts are sufficiently dense and hard for many applications.
In the WC-Co system, these sintering parts comprise hard phase paticles such as tungsten carbide, held together by a binder such as cobalt. Following the sintering process, the parts may contain voids surrounded by a mix of hard particles and binder in which the hard phase particles are spaced from each other by a distance less than the width of the void size.
For applications requiring still further densification, greater strength of the sintered part or better internal integrity, the parts may be further treated. Several types of treatments to accomplish this reduction in porosity are known in the prior art.
All of the previously described methods, although effective in reducing porosity and/or voids, suffer from problems in effectively producing a homogenous structure free from stress risers or internal stresses which can be detrimental to the performance of the material.
WC-Co alloys are ternary in nature due to the three constituents involved; mainly tungsten, carbon and cobalt. Because of this when the solidus or eutectic temperature is reached, all of the binder does not melt but rather this melting occurs over a range of temperatures. Also, some amount of solutioning of the WC occurs as the temperature is increased which may decrease the amount of solid remaining and increase the amount of liquid. The amount of liquid present at any given temperature in the three phase region where melting occurs, is very dependent on the exact composition of the WC-Co alloy under consideration.
FIG. 17 shows a vertical quasi-binary section of the WC-Co ternary phase diagram parallel to the C-W binary at 16% Co. This figure is from a paper by J. Gurland--Journal of Metals 6,285,1327 (1957). Similar phase diagrams will apply to other hard metals.
For WC-Co alloys to perform properly, they must avoid free carbon or carbon deficient phases in the microstructure. FIG. 17 shows that the useful area for WC-Co alloy of 16% cobalt is the WC+.gamma. field which ranges from 6.00-6.13 wt. % carbon. (.gamma. represents solid binder). A typical 16% WC-Co alloy would contain carbon in this range. When sintering such an alloy, some liquid phase should be present and this can be obtained in the WC+.gamma.+Liq field or in the WC+Liq field, i.e. all binder (.gamma.) has dissolved in the latter case. This would preferably be in the WC+Liq field as this would give the largest possible amount of liquid. The temperature at which the WC+liquid field is entered however, ranges from 1360.degree. C. to 1450.degree. C. depending on the exact carbon content of the alloy. At temperatures below this down to approximately 1300.degree. C., smaller and smaller amounts of liquid are available. For WC-Co alloys of other Co percentages, the temperature shown for the various fields will go up or down, depending on composition. In particular lower Co contents raise these temperatures and higher cobalt contents lower these temperatures. Also lower cobalt contents narrow the WC+.gamma. field and higher Co contents widen the WC+.gamma. field.
The amount of liquid phase and the amount of WC solutioning have a great effect on all of the methods used to reduce porosity and voids in WC-Co alloys. This effect is due to the changes in capillary and other forces upon the physical situation of the material at the time that the porosity reduction process is applied.
One such process if commonly referred to as hot isostatic pressing (HIP). In this process, the parts are placed in a pressurizable furnace and pressurized to approximately 5000 psi cold and then elevated to the solidus temperature and above. At this temperature (above approximately 1250.degree.-1350.degree. C.) the pressure is increased to above 10,000 psi due to the thermal expansion of the gas used to initially pressurize the system at room temperature. The primary advantage of HIP processing is to virtually eliminate all porosity within the part as well as greatly minimize or eliminate larger randomly spaces slits, holes or fractures which may be present in the part provided that such holes, slits, or fractures are not open to the surface. During the HIP process, as the parts are heated above the solidus, the binder, e.g. cobalt, begins to become molten and the spaces between the hard particles begin to form capillary passageways which are open to the voids in the part. In the absence of pressure applied to the part, the capillary force created by these passageways would prevent any molten binder contained within the part from entering the voids in the part. These capillary forces pulling the molden binder away from voids in the part can range from 20 to 1500 psi depending on the size of the flaws present and the size of the capillaries between the grains. These factors depend on the specific WC-Co alloy under consideration and the specific temperature for which the calculations are done.
During the HIP process, extremely high pressures (10,000-15,000 psi) are applied to the parts at a temperature below the sodius and subsequently the temperature is raised to above the solidus. When the solidus temperature is reaches, some liquid phase is formed. However, the structure will still be rather rigid since all of the binder will not have melted (WC+.gamma.+Liq field of FIG. 17). The high pressure applied to the parts at this time can then overcome the capillary forces and push the binder into any voids which might be present. This is well known in the art as "binder laking". This "binder laking" is one indication that some carbide users look for to be sure that a material was indeed HIP processed.
An example of such "binder laking" is shown in prior art FIG. 15 in which 16% WC-Co alloy part was subjected to the HIP process (1500.times. magnification). FIG. 15 (500.times. magnification) also shows a Hughes Tool Co. M.P.D. grade 168 after HIP processing. Large cobalt lakes are evident throughout the parts in both FIGS. 15 and 16. Although laking is preferable to porosity, it is much less desirable than a more homogeneous microstructure. Any discontinuity in a material including a "binder lake" will act as a stress riser when the material is subjected to stress and thus may shorten its useful life in a given application.
A further disadvantage of HIP processing is that due to the high temperatures and extremely high pressures used during the HIP processing, the previously known HIP equipment is extremely massive in construction and expensive to acquire. Also, HIP processing is a secondary process and requires that the parts be dewaxed and sintered in other equipment prior to being placed into the HIP equipment for that process to be effected.
Other methods wherein the sintering and hot isostatic processing may be carried out in one piece of equipment are disclosed in various prior art publications, some of which are discussed below.
Dr. Wolfgang Schedler, Reutte in Austrian Pat. No. 314,212 discusses a process for sintering alloys with a liquid phase in which, after reaching the eutectic temperature of the binder phase and a stage in the sintering shrinkage resulting in external sealing of still existing pores, the powder compacts are exposed to the isostatic pressure of an inert gas. He further states that "At about 50 vol % binder metal, a pressure of several bars is sufficient to achieve the effect of the invention," (hole closure) "while the final pressure should be about 200 bar or more below a binder metal of 10%.
In his example #1, a 25% cobalt material is heated to 1300 Degrees C. under vacuum and then subjected to 15 bar of argon pressure. The material is then further heated to 1320 Degrees C. Material prepared in this way performed 11/2 times better in a stamping application than did material prepared in a similar manner but without pressurization.
Similar results were achieved in example #2 with a 9% cobalt WC alloy processed at 1400 Degress C. and 100 bar of argon pressure.
Example #3: A 5% cobalt material using 1420 Degree C. and 150 bar of pressure.
Example #7: 10% cobalt WC material using 1390 Degress C. at 180 bar of argon pressure.
Example #9: An 8% cobalt WC material at 1350 Degrees C. and 130 bar of argon pressure.
In all of the above cases, especially #1 and #9, the material was pressurized at a temperature where the material would have minimal liquid phase present (FIG. 17) and to a pressure that would have overcome capillary forces and yet the bulk material would have considerable resistance to the macro movement of the microstructure due to the high proportions of the solid phases. This would be indicated by the statement that these relatively large, 100 to 200 bar (greater than capillary forces), pressures are needed to effect hole closure. Thus the voids and or porosity may, to some degree, be filled by the eutectic liquid and give relatively the same performance improvements as HIP processed material would indicate. Material processed in such a manner would also be subject to the same disadvantages as HIP processed material, mainly cobalt laking, and the fact that equipment capable of high temperatures and up to 3000 psi (recommended for materials less than 10% Co) are expensive to acquire and utilize a great deal of the gas used to pressurize which must be continually cleaned in order not to effect the chemical balance of the alloys being processed.
A. Hara and N. Yoshida in a Japanese Pat. No. Sho 46-9528 discuss a method similar to the Schedler patent wherein powders for the manufacture of cemented carbide are molded and sintered, and pressure is then applied to the sintered product using high pressure gas after the shrinkage is almost complete.
In the example, they discuss a 7 wt% Co WC alloy which is heated to 1300 Degrees C. for 2 hours under vacuum and then pressurized to 1000 Kg/cm.sup.2 (12000 psi). When pressurization is complete, the temperature is raised to 1400 Degrees C. and held for one hour to complete the processing. By treating such a material in this manner, pressurization again is carried out at a temperature where there may be minimal liquid phase available. Since however the pressure is very high, the small amount of eutectic liquid which may be present (if any) may be forced into any voids present by overcoming capillary action and thus effecting void closure. Again, this process may produce "cobalt lakes" which are preferable to voids but not as desireable as a more homogeneous structure. Also in this case, due to the very high pressures and the high temperatures used, the equipment necessary to accomplish this process is massive and expensive to acquire.
A somewhat different approach to densify carbide by pressurizing after sintering was taken by Johan Romp as described in U.S. Pat. No. 2,263,520. In this patent he describes a method of sintering carbide in a gaseous mixture of 85% N.sub.2 and 15% H.sub.2 or in 100% hydrogen for approximately 1 hour and then increasing the pressure by using a mixture of 85% N.sub.2 and 15% H.sub.2 to a pressure of 50 atm.
In his example I, he uses a 6% Co-WC alloy which he sintered at atmospheric pressure in an atmosphere of 85% N.sub.2 and 15% H.sub.2 at atmospheric pressure and 1450 Degrees C for 1 hour. At this point, the density was 14.5 gm/cu. Upon continued heating at 1450 Degrees C. and pressurizing to 50 atomospheres with the 85% N.sub.2 --15% H.sub.2 mixture and subsequently cooled at the increased pressure, the density was increased to 14.85 gm/cu.
In example II he used a mixture of 79% WC, 15% TiC, and 6% Co sintered for 1 hour at atmospheric pressure in hydrogen at 1500 Degrees C. At this time, the density was 11.1 gm/cu. On further heating at 1500 Degrees C. in a gas mixture of 85% N.sub.2 and 15% H.sub.2 pressurized to 50 stmospheres and subsequently cooled under this high pressure the specific weight is increased to 11.45 gm/cu.
Using this procedure, as can be seen from the rather low densities observed after initial sintering, there is a great deal of porosity in the parts at the end of initial sintering. These pores, although sealed from the surface due to the liquid phase sintering at the high temperatures used, would be filled with the gaseous mixture used during sintering. This would especially be true for the mixture of 85% N2 and 15% H2. Thus, even though the porosity is substantially reduced by pressurization, the gaseous mixture trapped in the pores will remain since the pressure inside the void will remain nearly equal to the pressure outside the part as the outside pressure is increased and the voids begin to shink. This will necessarily result in an increase in density. However, the number of voids in the part will remain the same although they will be smaller. This is pointed out in the body of the patent in that he states "the increased pressure is preferably maintained even during cooling to avoid any risk that when the gaseous pressure is decreased while the body is still hot and slightly plastic, the effect of the invention may be influenced detrimentally". This indicates that if the pressure (50 atm) was removed before the material was solid and able to withstand internal stresses, the gas pressure in the remaining pores would cause the pores to again expand and decrease the density. Thus, by using this procedure, the size of the pores are reduced thereby increasing the density but the number of pores may not be substantially reduced; therefore, porosity is not totally eliminated. This residual porosity is detrimental to the performance of the material although not as detrimental as the original porosity level (and pore size) might be.
A further disadvantage of this process is that the current furnaces used for sintering carbide contain graphite in the hot zone and typically the parts themselves are placed on graphite trays for sintering. If such a furnace contained H.sub.2 above about 1200 Degrees C., an equilibrium mixture of H.sub.2 and CH.sub.4 (formed by the reaction 2H.sub.2 +C solid=CH.sub.4) would be formed and this mixture would by definition have a carburization potential of 1 and thus add carbon to the WC-Co alloy such that it would exhibit free carbon in the microstructure which would be detrimental to its performance.
In order to sinter carbide in a hydrogen containing atmosphere in the presence of graphite, the parts are typically buried in A1203 sand which forms a local balanced atmosphere around the parts and protects them from carburization. Using this procedure, however, in a pressurizable furnace would greatly reduce the capacity of the equipment due to the volume of A1203 and sand used.
A further disadvantage is that equipment capable of high temperatures and a pressure of 50 atom (750 psi) is massive in construction and expensive to acquire.